This invention relates generally to monitoring systems for automobiles and other vehicles, and more particularly to signal generators of the type which incorporate sound transducers to produce audible chime-type sounds that indicate to the driver that a particular condition being monitored is in a certain mode or state.
In the past numerous chime generator circuits have been proposed and produced. Older model automobiles employed various types of buzzers or mechanical vibrators which are linked to switches disposed in various locations about the vehicle for reminding the driver that either the seat belts were not fastened, or that the headlights were inadvertently left on, etc. With the advent of the newer model cars, such mechanical type sound transducers have been replaced by electronic chime generator circuits which result in a significantly more pleasing sound than that produced by the older buzzer or vibrator types. The chime generators generally consist of an oscillator operating between 700 and 800 Hz which is modulated by a pulse having a relatively long rise and fall time, the pulse repetition frequency being on the order of the one cycle per second or so. The long rise and fall time produces the characteristic soft, ringing sound of a chime.
Conversion to the use of chime generators has not been without considerable sacrifice in complexity or cost, as compared to the prior warning systems that have been employed. In many installations, the chime generator was constructed as a plug-in module, and the electrical output of the generator was applied to a miniature electromagnetic speaker that was disposed within the module and constituted an integral part thereof. Needless to say, the cost of the miniature speaker was considerable; furthermore the sound produced by the chime generator lacked good quality and also emanated from the particular location of the module, which muffled the sound and sometimes resulted in poor audibility.
A number of other arrangements have been proposed and produced, as evidenced by the devices illustrated in U.S. Pat. Nos. 4,284,857; 4,232,287 and 4,160,132. In these patented arrangements, the transducer took the form of a flexible diaphragm of disk-like configuration, which was secured at its periphery, with an electromagnetic pole piece mounted beneath the diaphragm and disposed adjacent its center. A signal generator applied power to the coil surrounding the pole piece, causing the diaphragm to vibrate at an audio frequency, thus producing the desired sound. While such devices could be made to operate satisfactorily, they were relatively expensive to manufacture and mass produce, and brought about a number of their own problems which are peculiar to such devices. In this connection, reference is made to column 2 of Patent No. '132, line 42, where there is given an indication that the tolerances of certain gap dimensions associated with such devices must be kept within certain limits, falling which the quality of the sound produced suffered. See also column 4 of Patent No. '857, line 44 on; and column 2 of the same reference, the last few lines to the beginning of column 3.